Basically Water-Air delta is the difference in temperature between the air entering the radiator and the water leaving the radiator. It scales almost perfectly linearly, meaning that a radiator with a 10C delta dissipates twice as much heat as a radiator at a 5C delta.
Generally when designing a water-cooling loop you want to aim for a water-air delta in between 5 and 10C, however you can go lower if you want an extreme setup. Of course the closer you want your delta to 0C the more radiator power you need. For example, Fat_Italian_Stallion uses a total of 17x120mm radiators to cool his 920 and tri-sli GTX 480s. This gives him around a 3C delta. If he wanted to drop another 2C off his loop he'd have to triple the amount of radiators he has.
You can also lower your water-air delta by increasing your fan speed, but again you run into diminishing returns.
Check out Skinnee Lab's Triple Radiator
test for an awesome base of data if you want to calculate your own deltas.